1. Technical Field of the Invention
The present invention relates generally to a fuel injector to be installed in an internal combustion engine to spray fuel thereinto, and more particularly to such a fuel injector which has installed therein a fuel pressure sensor working to measure a change in pressure of fuel arising from the spraying of the fuel into the engine accurately.
2. Background Art
In order to ensure the accuracy in controlling output torque of internal combustion engines and the quantity of exhaust emissions therefrom, it is essential to control a fuel injection mode such as the quantity of fuel to be sprayed from a fuel injector or the injection timing at which the fuel injector starts to spray the fuel. For controlling such a fuel injection mode, there have been proposed techniques for monitoring a change in pressure of the fuel upon spraying thereof from the fuel injector.
Specifically, the time when the pressure of the fuel begins to drop due to the spraying thereof from the fuel injector may be used to determine an actual injection timing at which the fuel has been sprayed actually. The amount of drop in pressure of the fuel arising from the spraying thereof may be used to determine the quantity of fuel actually sprayed from the fuel injector. Such actual observation of the fuel injection mode ensures the desired accuracy in controlling the fuel injection mode.
For instance, in the case where a change in pressure of the fuel arising from the spraying of the fuel from the fuel injector (which will also be referred to as a fuel pressure change below) is measured using a pressure sensor installed directly in a common rail (i.e., a fuel accumulator), it will be somewhat absorbed within the common rail, thus resulting in a decrease in accuracy in determining such a pressure change. In order to alleviate this drawback, Japanese Patent First Publication No. 2000-265892 teaches installation of the pressure sensor in a joint between the common rail and a high-pressure pipe through which the fuel is delivered from the common rail to the fuel injector to measure the fuel pressure change before it is absorbed within the common rail.
The fuel pressure change, as produced at a spray hole of the fuel injector through which the fuel has been sprayed, will, however, surely attenuate within the high-pressure pipe. The use of the pressure sensor installed in the joint between the common rail and the high-pressure pipe, therefore, does not ensure the desired accuracy in determining the fuel pressure change. The inventors have study the installation of the pressure sensor in a portion of the fuel injector which is located downstream of the high-pressure pipe. Such installation, however, has been found to pose the problems, as discussed below.
The pressure sensor is typically made up of a sensing device and an electric circuit. The sensing device is usually designed to be sensitive to the distortion (i.e., elastic deformation) of an elastic member deformable as a function of the pressure of the fuel exerted thereon and output an electric signal indicative thereof. The electric circuit works as an amplifier to amplify the signal outputted by the sensing device. The fuel injector typically has formed therein a high-pressure path through which the fuel is to be delivered to the spray hole under high pressures and a metallic body within which an open/close mechanism working to open or close the spray hole is installed.
In use, the fuel injector is disposed near a combustion chamber of the engine, so that the metallic body will be subjected to high temperatures. Particularly, in diesel engines or direct injection gasoline engines in which the body of the fuel injector is installed in the cylinder head, the temperature of the body will be elevated up to as high as 140° C. This causes the electric circuit disposed in the body of the fuel injector to damage thermally, so that the performance and reliability in operation thereof will decrease. Specifically, the thermally damage results in a change in amplification of the output from the sensing device in the electric circuit.